Cook the onion in 1 tablespoon of the butter until the onion is soft but not brown.
In a large bowl combine the onion, mashed potatoes, bread crumbs, meat, 1/3 cup heavy cream, salt, egg, and parsley.
Mix them with a wooden spoon until well-blended.
Shape the mixture into small balls about 1 inch in diameter.
Put on a flat tray and chill for at least 1 hour before cooking.
Melt the remaining butter and the oil in a 12-inch skillet.
Fry the meatballs on all sides until they are done through, about 8 to 10 minutes.
Add more butter and oil if needed.
Transfer the finished meatballs to casserole or baking dish and keep them warm.
To make the sauce, remove the fat from the skillet.
Stir in the flour and cream.
Stir constantly as the sauce comes to a boil and becomes thick and smooth.
Pour it over the meatballs.